New Relic config

Config file updated for the new gem version.

Addresses things like this too:

```
# When "true", the agent collects performance data about your
# application and reports this data to the New Relic service at
# newrelic.com. This global switch is normally overridden for each
# environment below. (formerly called 'enabled')
monitor_mode: true
```

Also fo:

- Easier to read/customise.
- Logging level to info rather than debug
- Ability to use in staging